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Re: [Qemu-devel] [PATCH v5 12/21] block: define get_block_status return

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v5 12/21] block: define get_block_status return value
Date: Tue, 06 May 2014 15:20:42 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0

Il 06/05/2014 15:08, Kevin Wolf ha scritto:
This is true, because bdrv_get_block_status does not ask the backing
file whether it is zero in the backing file.  Thus, reading from the
backing file does not guarantee to read zeroes.

Aha, this paragraph is the key. :-)

I assumed that bdrv_get_block_status() does ask the backing file, but
apparently it doesn't. The current meaning of BDRV_BLOCK_ZERO is
therefore not simply "sectors read as zero", but "sectors read as zero
and this is determined by bs itself (mostly, except for the backing file


Is it a good idea to rely on this or would it be reasonable for
bdrv_get_block_status() to look at the backing file for returning ZERO
sometime in the future? I think that could be useful information in some

It is reasonable, for example SCSI emulation could use it. However, it would be a different API (like bdrv_is_allocated_above). It would be very similar to get_block_status() in qemu-img.c.


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